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2.2 Historical Aspects 37

In heterogeneous catalysis, the catalyst provides a surface on which the reactants are
adsorbed. The chemical bonds of the reactants become weakened on the catalytic surf ace
and new compounds are formed. These compounds (products) have weaker bonds with the
catalyst and consequently are released. An example of heterogeneous catalysis is the
industrial synthesis of ammonia, which requires solid catalysts to obtain significant rates
of reaction between nitrogen and hydrogen:

)
t
n
f
r
g
s
p
i
ad
)
N ( N ( (adooo ntrocattsu i en o g n y s al rface)
s
2 2
N ( 2N(a (of nitroge
dissoc
tion
n)
ds)
ds)
a
i
a
2

ads)
g)
H ( H ( (adsorptionof hydrogenoncatals yst urface)
2 2
H ( 2H(a (of hydroge
dissoc
tion
i
n)
a
a
ds)
ds)
2
N(ads) 3H N ( (reac tion tofaa orm mmonia d) dsorbe
)
s
(ad
H
ad
)
s
3
NH (a N ( (deof a
orption
ds)
s
g)
H
mmonia
)
3 3
It has to be noted that the adsorption of reactants is generally not uniform across the cata-
ace.
lyst surfAdsorption, and therefore catalysis, takes place mainly at certain f orable v a
locations on a surface called actie sites. In en catalysts are essen- v , vironmental chemistry
tial for breaking down pollutants such as automobile and industrial e xhausts.
2.2 HISTORICAL ASPECTS
2.2.1 Adsorption
The first known use of adsorption was made in 3750 B.C. by Egyptians and Sumerians
who used charcoal for the reduction of copper zinc, and tin ores for the manufacture of
,
bronze. Around 1550 B.C., Egyptians applied charcoal for medicinal purposes, whereas
around 460 B.C., Hippocrates and Pliny introduced the use of charcoal to treat a wide
range of infections. Around the same age, Phoenicians used charcoal filters to treat drink-
ater ing w. So, this must hae been the first use of adsorption for en v vironmental purposes.
In 157 B.C., Claudius Galen introduced the use of carbons of vgetable and animal origin e
to treat a wide range of complaints.
These early applications of adsorption were based on intuition and not on a systematic
study. It was in 1773 that Scheele made the first quantitatie observations in connection v
with adsorption, whereas F. Fontana in 1777 reported his experiments on the uptake of
gases from charcoal and clays. However, the modern application of adsorption is attributed
to Lowitz. Lowitz used charcoal for the decolorization of tartaric acid solutions in 1788.
The next systematic studies were published by Saussure in 1814. He concluded that all
types of gases can be taken up by a number of porous substances and this process is
v accompanied by the eolution of heat (Dabro 2001). wski,